This invention relates to a method of mounting printing blocks on form cylinders of a flexographic printing machine for multicolor printing in correct positions determined by a mounting pattern, which is secured to a mounting cylinder or mounting table.
For mounting printing blocks in correct positions on form cylinders it was previously usual to adhesively join a registration sheet to a mounting cylinder and to use the registration sheet as a reference sheet for checking the position of the printing block. Exact registration lines for the print had previously been drawn by hand on the registration sheet. These registration lines, i.e., the contour lines of the printing block, were then transferred by means of a mirror to the form cylinder. Thereafter, the printing block was adhesively joined to the form cylinder by hand in the pattern formed on the form cylinder by the registration lines which had been transferred by means of a mirror. Ink was then applied by hand to the printing block and a proof was printed on the registration sheet. When the printing block had been finally aligned, the form cylinder carrying the printing block was removed and the form cylinder for the next color was hung into the mounting device. When the registration lines had again been transferred by means of the mirror, the printing block was secured to that form cylinder.
The registration lines drawn on the registration sheet by hand differed in thickness. These inevitable inaccuracies regarding the line thickness were transferred by the mirror to the form cylinder so that it was not possible to mount and secure the printing blocks with exactly straight edges. The inaccuracies increased with the distance from the mirror to the form cylinder. Owing to the fine line screens on the printing blocks, it is absolutely essential that all printing blocks used for the multicolor printing are secured to the form cylinders with exactly straight edges. For this reason a partial offset of the screen line intersections relative to each other was inevitably involved in the known method of mounting the printing blocks and the printed colors were blurred. As the inaccurate prints obtained from the printing blocks which had been mounted on the form cylinders by the known method were due to the fact that the printing blocks had been mounted in an oblique position, it was not possible to improve the print by an automatic registration control.